Data storage subsystems used in data processing systems commonly comprise a device controller connected to one or more storage devices on which customer data is retained. These storage devices are commonly direct access storage devices e.g. disk drives. In recent years, such storage subsystems have become more sophisticated and a number of different subsystem architectures have been proposed.
U.S. Pat. No. 4,825,406 (assigned to Digital Equipment Corporation) describes a secondary storage facility which employs serial communications between a device controller and attached disk drive. The serial link consists of four lines which are unidirectional bit serial channels. One of the lines carries write and command data to the drive, another carries read and response data from the drive to the controller. The remaining two lines carry signals for use in coordinating and synchronising transmissions. The device controller communicates with a host processor to which it is attached via a bus.